It has been assumed for a number of years that testosterone was the main androgen responsible for the maintenance for spermatogenesis as well as male accessory structures and secondary sexual characteristics. In view of the fact that some of the metabolites of testosterone are capable of maintaining spermatogenesis, other naturally occuring metabolites needs to be identified afd tested for biological activity. In the proposed studies, attempts will be made to identify the unknown testosterone metabolites by in vitro incubation techniques. The biological activity of the metabolites will be judged by their ability to maintain spermatogenesis in non-regressed hypophysectomized ratss, as well as their ability to reinitiate spermatogenesis in post-hypophysectomized regressed testis. In each instance, attempts will be made to quantify the maintenance of spermatogenesis. Because of the possibilityof differential metabolism of androgens by various organs of the body, a number of different routes of administration will be utilized. Depending on the objectives of the experiment, hormones will be delivered by subcutaneous injections, silastic capsule implants in the testes, intratesticular injections or by continuous intratesticular infusion.